1. Technical Field
The present invention relates to a flat panel display and, more particularly, to an organic electroluminescent display and a method for fabricating the same, capable of improving luminous efficiency and color reproduction by differentiating thicknesses of anode electrodes of respective R, G and B unit pixels.
2. Prior Art
Generally, an organic electroluminescent display (OELD) is classified into a front surface emitting display and a rear surface emitting display depending upon the surface of luminescence of light. On the basis of a substrate, in case of the rear surface emitting OELD, a light from an electroluminescent layer is emitted and passes through the substrate, while in the case of the front surface emitting OELD, light from the electroluminescent layer is emitted without passing through the substrate.
The efficiency of the rear surface emitting structure is determined depending upon optical characteristics of a reflection film and a transmissive anode electrode, and electrical characteristics of an organic thin-film layer including an electroluminescent layer. A hole transporting layer is formed so as to be thicker than an electron transporting layer since maximum constructive interference in optical characteristics is generated at a thickness of ¼ wavelength of light emitted. The mobility of the hole transporting layer is faster than that of the electron transporting layer in terms of the electrical characteristics. Therefore, the thickness of the electroluminescent layer presenting the maximum efficiency is determined when a full color OELD of the rear surface emitting structure is manufactured.
On the other hand, the thicknesses of the hole transporting layer, the electroluminescent layer and the electron transporting layer located between a reflective anode electrode and a transflective cathode electrode for measuring the optical thickness, and the electrical thickness in the front surface emitting OELD, are determined differently from the rear surface emitting OELD.
There have been prior attempts to obtain the maximum efficiency and the highest color purity by controlling the thicknesses of a hole injecting and transporting layer, an electroluminescent layer and an electron transporting layer making up the organic thin-film layer interposed between the anode electrode and the cathode electrode. Japanese Patent Registration No. 2846571 has disclosed technology in the area of the rear surface emitting organic electroluminescent display capable of obtaining a high color purity and efficiency by setting an optical film thickness of the anode electrode, the cathode electrode and the organic thin-film layers between the anode and cathode electrodes to achieve a peak in the strength of light emitted from the electroluminescent layer. Further, Japanese Laid-open Patent Publication No. 2000-323277 has disclosed technology in the area of the rear surface emitting organic electroluminescent display capable of obtaining a high efficiency and color purity by differently forming the thickness of thin-film layers, except for the electroluminescent layer, among the organic thin-film layers interposed between the anode electrode and the cathode electrode, depending upon the R, G and B unit pixels.
However, the front surface emitting organic electroluminescent display has a problem in that, although the thickness of the thin-film layers is set to ¼ wavelength of a desired light, it is difficult to obtain a desired efficiency and color purity since the electroluminescent layer is located between reflection sections of the reflective anode electrode and the semitransmissive cathode electrode.
On the other hand, in the front surface emitting electroluminescent display, U.S. patent application assigned Ser. No. 10/385,453 entitled “Organic Electroluminescent Device Employing Multi-Layered Anode”, by Kwanhee Le, filed in the United States Patent & Trademark Office on the 12th day of Mar. 2003, has disclosed technology capable of improving luminescence characteristics by forming an anode electrode of a multi-layered structure.
Anode electrodes of the respective R, G and B unit pixels are formed on an insulating substrate. The anode electrodes include a first anode and a second anode. A pixel defining layer is formed to expose portions of the anode electrodes, thereby forming apertures of the respective R, G and B unit pixels. Organic thin film layers of the R. G, and B unit pixels, including R, G, B electroluminescent layers, are formed on the anode electrodes of the R, G and B unit pixels, respectively, in the apertures. A semitransmissive cathode electrode is formed on the entire surface of the substrate.
The front surface emitting organic electroluminescent display has formed thereon anode electrodes with a 2-layered structure, employing a first anode electrode as a metal film having a high reflectivity, and a second anode electrode as a metal film capable of conforming with a work function, thereby improving luminous efficiency by increasing reflectivity and a hole injecting characteristic.
However, in the front surface emitting organic electroluminescent display, all of the second anode electrodes of the respective R, G and B unit pixels have the same thickness. Therefore, it is impossible to obtain desired color reproduction and efficiency, since the first anode electrode with a good reflectivity and the semitransmissive cathode electrode have respective lengths of optical constructive interference which are different from each other.